Abstract：As an emerging medical imaging technology, magnetic particle imaging (MPI) possesses numerous unique advantages and holds broad application prospects in multiple fields. However, currently, MPI is confronted with a critical problem: its image resolution fails to meet the clinical demands for in-vivo medical human imaging. In theory, increasing the particle size of magnetic nanoparticles can enhance the MPI resolution. Nevertheless, in practical measurements, a larger particle size intensifies the relaxation effect. Due to this relaxation effect, magnetic particles cannot reach the magnetization steady-state under the constantly changing sinusoidal excitation, exerting a non-negligible negative impact on imaging. Therefore, it is necessary to improve the MPI system to enable the magnetization of magnetic particles to reach a steady state and eliminate the influence of the relaxation effect. In view of this, a pulsed magnetic field excitation method is put forward in this paper. Firstly, to accurately describe the magnetization change process of magnetic particles, the Langevin magnetization theory was combined with the Debye relaxation theory to construct a theoretical magnetization model of magnetic particles. Simultaneously, an open-structure MPI system simulation model is established, which encompasses key components like excitation coils, detection coils, compensation coils, gradient coils, and focus coils. The gradient coil adopts an improved rectangular gradient coil to enhance the accuracy and uniformity of the field free line (FFL). Secondly, a pulsed excitation method is proposed. The area under the decaying voltage curve is chosen as the imaging parameter for pulsed-excitation MPI. By passing current through the excitation coil in the simulation model, a pulsed magnetic field was generated. This pulsed magnetic field contains quick changing and constant components, capable of exciting magnetic particles to generate signals while enabling magnetic particles to reach the magnetization steady-state. Subsequently, simulation experiments were carried out using the established simulation model.Under the same conditions of excitation frequency, amplitude, and particle size, the magnetization signals of magnetic particles under pulsed and sinusoidal excitations are compared. The results indicate that pulsed excitation can make the magnetization of magnetic particles reach a steady-state. Meanwhile, the signal point-spread functions under different excitations were also measured. The resolution was evaluated by the full width at half maximum (FWHM) of the signal point-spread function. The experimental results show that the FWHM of the signal point-spread function of pulsed-excitation MPI is 84% of that of sinusoidal-excitation MPI, indicating an improvement in resolution. Finally, A line-source imaging experiment is carried out using the x-space method to further visually demonstrate the differences in the imaging resolution of MPI under different excitations.The single-line-source imaging results demonstrate that pulsed-excitation MPI has fewer imaging artifacts and no imaging offset occurs. The The double-line-source imaging demonstrates that the pulsed excitation MPI has a stronger resolving ability for two adjacent line sources with an interval of 1 mm. The results of the simulation experiments and imaging experiments indicate that modifying the sinusoidal excitation magnetic field of traditional MPI to pulsed excitation can enable the magnetization of magnetic particles to reach a steady state, thereby eliminating signal lag and improving the resolution of MPI.
李美凝, 杜强, 柯丽. 脉冲激励磁性纳米粒子的磁化稳态方法[J]. 电工技术学报, 2026, 41(1): 37-45.
Li Meining, Du Qiang, Ke Li. The Method of Magnetization Steady State of Magnetic Nanoparticles by Pulsed Excitation. Transactions of China Electrotechnical Society, 2026, 41(1): 37-45.
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